The term “gas-operation system” in the context of firearms generally refers to a system of operation used to provide energy to operate auto-loading firearms. In gas-operation, a portion of high pressure gas from the cartridge being fired is used to power a mechanism to perform the following operations: (1) extracting the spent casing, and (2) chambering a new cartridge. For example, in a firearm based on the AR-15 platform, there is usually a small gas port, or small hole, in the barrel that vents gas with every shot. In a “direct impingement” gas system, the vented gas travels through a gas block and gas tube into the receiver, where the gas powers the bolt carrier group (BCG) and auto-cycles the next round. In a “piston-operated” or “piston-stroke” system, the vented gas acts upon a face of a piston, which in turn moves the BCG to extract the spent casing and chamber a new cartridge.
Usually, the gas system is tuned to one specific ammunition of certain caliber to work the best in a typical environmental condition and certain firearm configuration. This is what is called a fixed gas system. The advantage of this system is that it is light weight, has fewer parts, and is very reliable if the working condition and firearm configuration are correct. A typical example of such system is on AR-15 rifles, a civilian variant of the military version of M16/M4 rifle. However, as environmental condition changes and/or as the firearm configuration changes, or simply because the firearm is not assembled right, the firearm could experience what is called an over-gas situation or an under-gas situation.
The over-gas situation tends to happen when the firearm is used in conjunction with a silencer, or some kind of recoil reduction muzzle device, such that the back pressure generated by such device would add the gas pressure to the gas system. As such, the bolt carrier group tends to move back faster with more energy. Given a weak extractor spring, the extractor may run over the rim of the spent casing and leave it stuck inside the chamber, thus resulting in jamming the firearm, when the BCG travels back and reloads the next round. Moreover, a so-called “felt recoil” may result, and the firearm may be put under more stress and hence its service life may be shortened.
The under-gas situation tends to happen when the firearm gets dirty, or when the environment is very cold such that the system does not have enough gas energy to recycle the firearm. As such, the BCG either may not move back enough to either eject the spent casing or may fail to load the next round, thus resulting in jamming the firearm.